During the fabrication of photovoltaic devices, layers of semiconductor material can be applied to a substrate with one layer serving as a window layer and a second layer serving as an absorber layer. In addition to the semiconductor layer (the window and absorber layers), photovoltaic modules, devices, or cells, can include multiple layers (or coatings) created on a substrate (or superstrate). For example, a photovoltaic device can include a barrier layer, a transparent conductive oxide layer, a buffer layer, and a semiconductor layer formed in a stack on a substrate. Each layer may in turn include more than one layer or film. For example, a semiconductor window layer and a semiconductor absorber layer together can be considered a semiconductor layer. Additionally, each layer can cover all or a portion of the device and/or all or a portion of a layer or a substrate underlying the layer. For example, a “layer” can include any amount of any material that contacts all or a portion of a surface. Cadmium telluride has been used for the semiconductor layer because of its optimal band structure and a low cost of manufacturing.
In the manufacture of a photovoltaic device, the absorber layer is deposited on a substrate. This may be accomplished by vaporizing the semiconductor and directing the vaporized semiconductor towards the substrate surface such that the vaporized semiconductor condenses and is deposited on the substrate, forming a solid semiconductor film. The method by which the absorber layer is deposited or formed on the substrate may have an effect on the performance of the photovoltaic device. Improving the characteristics of the absorber layer may improve efficiency thereof, and therefore the efficiency of the photovoltaic device. Furthermore, it is desirable to improve a band alignment at the interface of the absorber layer and adjacent layers to improve the efficiency of the photovoltaic device.
One way to improve the efficiency of an absorber layer is to alloy materials deposited as the absorber layer and/or to alloy the absorber layer and the back contact buffer layer at an interface thereof. Using vapor transport deposition (VTD) or close-spaced sublimation (CSS), improvement to an absorber layer by forming an alloy may be accomplished. However, the compounds used to form the absorber layer may have vastly different vapor pressures that lead to composition variation over time. For example, composition variations may occur due to faster sublimation of one of the compounds, a difference between atomic weights of the compounds, and the annealing temperatures used by known processing equipment may not be sufficiently high to vaporize certain compounds. Furthermore, at VTD and CSS process temperatures, some compounds have solubility gaps which prevent the alloy from forming, and if cooling is not fast enough, precipitation from the alloy may occur.
Alloyed compounds for the absorber layer and/or the back contact or front contact buffer layers may also be generated by coating the substrate within a coating device or by annealing in an oven. The temperatures required to anneal certain compounds to obtain an alloy may exceed 600° C. which may have an undesired effect on a glass substrate. Steps of alloying in the coating device or by annealing in an oven have the same solubility gap and precipitation concerns noted above. Annealing the entire absorber layer at high temperatures, such as in an oven, may also result in sublimation of the compounds forming the absorber layer.
A crystallized back contact buffer layer also improves operation of the photovoltaic device. Depositing the back contact buffer layer at high temperatures to crystallize the back contact buffer layer improves band alignment and photovoltaic device operation. The back contact buffer layer may also be annealed in an annealing oven. However, high temperature deposition of the back contact buffer layer comes and/or using an annealing oven comes at a high capital cost.
It would be desirable to develop a method of improving the operation of a photovoltaic device by improving the interfaces of various layers of a photovoltaic device, for example, by improving a band alignment at the interface of the absorber layer and the back contact buffer layer or annealing the back contact buffer layer using a more efficient method.